Method of recovering crude oil from a subsurface formation



m vulv vw um. MMMM n March 17, 1970 LEHNER ETAL 3,500,917

METHOD OF RECOVERING CRUDE OIL FROM A SUBSURFACE FORMATION Filed Aug. 28, 1968 FIG. 2

z u m a INVENTORSI F. LEHNER F. VAN DAALEN W. VAN DER KNAAP TH E-lR' ATTORNEY Int. CI. 1 121!) 43/22 US. Cl. 166272 Claims ABSTRACT on THE DISCLOSURE A method of recovering crude oil having a viscosity above 50 cp. from a subsurface formation having an oilsaturated zone overlying a water-saturated zone wherein a solvent/ aqueous fluid mixture is injected into the watersaturated zone, said solvent having a density smaller than the density of the crude oil.

The present invention relates to a method of recovering crude oil from a subsurface formation containing crude oil having a viscosity higher than 50 cp., which oil does not flow at economic rates through the pore space of the formation under its own energy, even if pressure differences are raised over wells penetrating into said formation,

The invention relates in particular to a recovery method for the recovery of crude oil from a formation having the oil-saturated part of the pore space thereof located above a water-saturated part. An example of such formation is the Peace River tar sands in Canada. At least one well suitable for the injection of fluids into the formation and at least one well suitable for the production of fluids from the formation penetrate into the formation, such that the communication between the interiors of the wells and the pore space of the formation is in the water-saturated part of the pore space of the formation.

SUMMARY OF THE INVENTION An object of the present invention is to provide a recovery method for the recovery of crude oil having a viscosity higher than 50 cp. from formations having a watersaturated lower zone, above which zone there is located an oil-saturated zone, which method includes applying a solvent for decreasing the viscosity of the oil.

A further object of the invention is to provide a method of recovering oil having a viscosity higher than 50 cp. from an oil-saturated zone overlying a water-saturated zone, wherein use is made of a solvent which is distributed over the lower face of the oil-saturated Zone so as to decrease the viscosity of the oil present at the lower side of the oilsaturated zone,

Still another object of the invention is to provide a recovery method for the recovery of viscous crude oil from a subsurface formation having a water-saturated lower zone, wherein use is made of hot fluids, which method obviates the risk of plugging the formations by crude oil which has during the heating-up period of the formation first been heated and made flowable, but cools down and solidifies in those parts of the formation which are still cold, thereby blocking the passageway through the formation to the production well.

Another object of the invention is to provide a method of recoverying viscous oil having a viscosity above 50 cp. by means of thermal energy, wherein fluids are applied which have the property to decrease the viscosity of the oil independent of the temperature of the applied fluid.

Still. a further object of the invention is to provide a tates Patent O ice 3,500,917 Patented Mar. 17, 1970 method for recovering crude oil having a viscosity higher than 50 cp. from a formation having the oil zone located above a water zone, wherein the recovery efliciency is greatly increased with respect to the prior art methods.

According to the invention, there is simultaneously injected into the water-saturated zone of the pore space of the formation containing crude oil having a viscosity above 50 cp., an aqueous fluid which in its liquid phase has a density greater than the density of the crude oil and a. solvent suitable for dissolving crude oil and having in its liquid phase a density which is smaller than the density of the crude oil. Fluids comprising crude oil are produced from the formation during this injection.

The aqueous fluid may comprise water. The aqueous fluid may also have a temperature which is higher than the original formation temperature, and may at least partially consist of steam.

A suitable solvent for this purpose is in principle any liquid miscible with the crude oil and having a density smaller than the density of the crude oil at prevailing formation temperature and pressure. As examples of such solvent there may be mentioned liquid natural gas, liquefied petroleum gas, kerosene, and light crude oil.

DESCRIPTION OF THE DRAWING These and other objects of this invent on will be understood from the following description taken with reference to the drawing wherein: f1..-

FIGURES l, 2 and 3 are cross-sectional views showing in a schematic manner a section over an oil-containing formation, this section being taken over the locations where an injection well and a production well penetrate said formation, and illustrating three consecutive stages of the recovery methods according to the present invention.

The oil-containing formation 1 (FIGURE 1) is bounded by a caprock 2 and a base rock 3 which are (or at least the parts thereof adjoining the formation 1) substantially impermeable to the liquids present in the formation 1. An injection well 4 and a production well 5 penetrate this formation. The pore space of the formation 1 is divided into two zones, the first zone being the oilsaturated zone 6 of the formation in which the poresfare substantially (that is, for at least vol. percent) filled with oil, and the second zone being the water-saturated zone 7 which is for at least 80 vol. percent thereof filled with water. The viscosity of the oil is higher than 50 cp., and the oil cannot be produced economically when opening one or both of the wells 4 and 5 penetrating into the formation 1. It will be appreciated that the interface8 of the two zones 6 and 7 may have a substantial thickness and be formed by a formation pore space in which the oil saturation is lower than in the overlying zone and the water saturation is lower than in the underlying zone.

The wells 4 and 5 penetrate into the formation 1 to a level which is near the interface 8 and preferably therebelow, and are equipped with the means known per so which are required to make these wells suitable for the use as an injection well and a production well, respectively. The well 4 is provided with means which allow the injection of two fluids. These two fluids may be passed down through the well through a single tube being the casing or an injection tubing which is suspended in the casing, which tubing may be provided with insulating means for preventing heat loss if hot fluid is passed'therethrough. The fluids, if such is considered desirable, may also be injected separately down the well 4, e.g., bypassing one fluid through an injection tubing and the other fluid through the annular space between the injection tubing and the casing, or by using separate injection tubings. The well 4 may also be substituted by a pair of twin wells,

One of the fluids which are injected in the abovedescribed manner into the formation 1 substantially consists of water, which is injected into the top of the well 4 at ambient temperature. The other fluid substantially consists of a solvent suitable for dissolving the crude oil present in the formation zone 6.

Any solvent suitable for the purpose may be applied. As examples of such solvent there are mentioned liquid natural gas, liquefied petroleum gas, kerosene, and light crude oil.

The solvent having a density smaller than the density of water will, after being injected into the zone 7 of the formation 1, flow in the direction of the production well and be on top of the water which is simultaneously injected therewith. Consequently the solvent Will sweep over the lower surface of the oil zone 6, thereby decreasing the viscosity of the oil in the lower face of this zone6, which oil under influence of the pressure difference created between the wells 4 and 5 by the solvent/water flow 9 (FIGURE 2), is displaced toward the well 5, from which it is recovered by suitable means.

it will be appreciated that the fluid as recovered via the well 5 is treated for separating the solvent as well as the oil therefrom. The solvent, and if desired also the water as recovered via the well 5, is returned to the injection well 4 and reinjected therein. A fresh amount of water may further be supplied to the well 4 for making up for the volume of oil recovered from the zone 6. Thus the solvent floating on top of the water in the zone 9 will remain. in contact with the lower face of the oil zone 6.

If desired, the communication between the well 5 and the formation 1 may be arranged at a level higher than shown in the drawing. Thereby the amount of water recovered from the formation may be reduced. Any other method for reducing water coming at the well 5 may be applied.

When the interface 8 between the oil-saturated zone 6 and. the Water-saturated zone 7 is shifted to a higher level as a result of the removal of crude oil from the lower parts of the oil-saturated Zone 6, the communication between the well 5 and the formation 1 may also be shifted to a higher level. Such may, e.g., be done by plugging the perforations in the cemented casing of the well 5 and reporforating the casing at a higher level. The same applies for the injection well 4 if such should be considered desirable.

After prolonged injection of the solvent and the water via the well 4, the solvent will penetrate into the lower part of the oil zone 6 as indicated by the arrows 10 in FIGURE 3. This results in a layer of oil of appreciable thickness which. has a reduced viscosity and moves as a whole to the production well 5 under influence of the pressure difference existing between the Wells 4 and 5.

If desired, the solvent and the water may be injected into the formation 1 in the form of an unstable emulsion. This emulsion may either be formed at the surface, or during the passage of the water and the solvent through the well 4 or during entry of the water and solvent into the formation 1. The instability of the solvent/water emulsion ensures that the emulsion breaks gradually during the passage thereof through the zone 9. Since the breaking of the emulsion does not occur suddenly, the solvent is evenly distributed over the lower face of the oil zone 6.

Any suitable emulsifying agent may be applied for this purpose. This agent will be adsorbed by the formation rock during passage of the emulsion through the zone 9, which will cause a gradual breakdown of the emulsion.

It will be appreciated that the density of the solvent/ water emulsion will be smaller than the density of the original formation Water present in the zone 7. Thus the emulsion will flow toward the well 5 through the zone 9 as shown in FIGURE 2 of the drawing.

If the crude oil contained in the oil zone 6 cannot be produced at an economic rate by the method as described in the above example (which may be due to the viscosity of the oil and/ or the permeability of the formation), the aqueous fluid as introduced into the formation 1 via the well 4 is not injected at ambient temperature, but is heated before its injection to a temperature such that it enters the pore space of the formation 1 at a temperature which is higher than the original formation temperature, i.e., the temperature of the formation 1 just before the present process is applied.

The hot aqueous fluid has in its liquid phase (which phase exists under the pressure and temperature conditions of the formation when the oil is recovered therefrom) a density which is higher than the density of the crude oil. The hot aqueous fluid which may comprise hot water and/or steam, travels through the formation 1 from the injection well 4 toward the production well 5. If steam is applied, this steam will condense before reaching the production well 5. Since the viscosity of the contents of the pore space in the water-saturated zone 7 is smaller than the viscosity of the contents of the pore space of the oil-saturated zone 6 of the formation, the hot fluid will, after leaving the injection well 4, pass through the pore space of the water-saturated zone 7 of the formation 1, even if the outlet opening of the well 4 should be slightly above the interface 8.

Since the density of the hot aqueous fluid is smaller than the density of the formation water present in the water-saturated zone 7, the hot aqueous fluid will sweep along the lower boundary of the oil-saturated zone 6 of the pore space, and pass over the top of the water in the watersaturated zone 7 of the pore space. Thus the lower part of the viscous oil present in the oilsaturated zone in the neighborhood of the well 4 will be heated, and move, since its viscosity is reduced, toward the production well 5 under influence of the pressure difference existing across the wells 4 and 5 by the injection of fluid into the well 4. If desired, this pressure difference may be increased by lowering the pressure within the well 5 by pumping.

In the initial stage of the recovery process, the leading end of the hot aqueous fluid tongue which is penetrating into the formation 1 along the plane 8 of the interface between the zones 6 and 7 of the formation, cools down to formation temperature after having given off its heat content. Hereby the oil which is moved into the formation part which is still at original formation temperature, will also cool down with the result that if no solvent was present, it would become immobile, thereby urging the fluid flow to a lower depth in the formation. By thus bending the flow of fluid from the original interface plane 8, the heat transmission between the hot fluid and the high viscous crude oil in the part 6 of the formation would be hampered, which would result in a low thermal efliciency.

This latter, however, is prevented in the present method since a solvent suitable for dissolving the crude oil is simultaneously injected with the hot aqueous fluid. As this solvent has in its liquid state (which is at temperature and pressure conditions prevailing in the formation during the present recovery process) a density which is smaller than the density of the crude oil (and thus a density' which is also smaller than the density of the hot aqueous fluid), this solvent will tend to enter the 011- saturated zone 6 and thereby pass through the formation ll directly below the oil-saturated zone 6 of the formation ll, thereby dissolving any oil (and decreasing the viscosity thereof) which has been pushed into the still cold parts of the formation 1. Plugging of the water-saturated zone 7 and bending off of the hot fluid stream to deeper formation layers is thus prevented, and the heat content of the hot fluid is used to its maximum effect.

In FIGURE 2 of the drawing, the situation is displayed when the hot aqueous fluid and solvent which are simultaneously injected via the well 4 have broken through in the well 5, from which they are recovered, together with the heated crude oil which has been loosened from the bottom of the oil-saturated zone 6 of the pore space. The passageway or zone 9 extending between the wells 4 and 5 is thus filled with water, solvent and crude oil with reduced viscosity.

A further advantage of the solvent is illustrated in FIGURE 3. Here the solvent, after prolonged injection thereof together with the hot aqueous fluid, enters the oil s'aturated pore space 6 in the direction of arrows 10, thereby cooperating with the hot fluid to reduce the viscosity of the crude oil present in the lower parts of the oil-saturated zone 6 of the formation 1. Consequently a layer of crude oil having the viscosity thereof reduced will move toward the production well 5, this layer having a greater thickness than the one which would result from the use of hot aqueous fluid without a solvent added theteto. F

An even distribution of the solvent over the lower surface of the oil-saturated zone 6 may be obtained by injecting the solveni't and the hot aqueous fluid in the form of an unstable emulsion. Breaking of the emulsion results in a vertical separation of the components thereof, the solvent moving inupward direction and coming into contact with the crude oil in the zone 6. By controlling the instability of the emulsion, the point where the emulsion breaks down can be varied. In the later stages of the process, this point may be shifted toward the productiori well 5.

The solvent for such a solvent/water emulsion may in principle be formed by any liquid miscible with the crude oil and having a density lower than the density of the crugie oil at prevailing formation temperature and pressure. As previously stated, examples of such solvent are liquid naturalgas, liquefied petroleum gas, kerosene, and light crude oil. Any suitable emulsifying agent may be applied. The agent will be adsorbed by the formation rock when the emulsion passes therethrough, which will cause a gradual breakdown of the emulsion.

It will be appreciated that the fluid as produced via the production well 5 is treated for separating the solvent as well as the oil' therefrom. This solvent is returned to the injection well and re-injected thereto together with hot aqueous fluid. This recirculation of solvent may be repeated as many times as desirable. If desired, the water produced via .the well 5 may be reinjected into the well 4 together with a fresh amount of water.

Since the viscosity reduction of the crude oil is not only obtained by the solvent but also by the hot aqueous fluid, whereas the solvent is in particular suitable for removing immobile crude oil which would hamper the flow of the hot aqueous fluid to the production well in the initial stage of the process, the injection of solvent may be interrupted after the lower layers of the oil-saturated zone 6 have obtained a sufficiently low viscosity by the combined action of the solvent and the hot aqueous fluid. If desired, the recovery may then proceed by injecting hot aqueous fluid only, or by periodically injecting solvent either in the form of slugs simultaneously with the hot aqueous fluid or emulsified with this fluid. The advantage as described in connection with FIGURE 3 will then also be obtained, though with a somewhat smaller efiiciency than when the solvent is continuously being applied.

When the interface 8 between the oil-saturated zone 6 and the water-saturated zone 7 is shifted to a higher level by the removal of crude oil from the lower parts of the oil-saturated zone, the openings through which the wells 4 and 5 communicate with the formation pore space may also be raised to a higher level. Such may, e.g., be performed by plugging the perforations in the cemented casings of the wells and reperforating these casings at a higher level which is near the new interface 8. -It will be appreciated that the communication between the well 4 6 and the pore space of the formation 1 need not be arranged at the same level as the communication between the well 5 and the pore space of the formation 1.

The aqueous fluid may be heated to the desired temperature in any suitable manner. If necessary, the fluid is pretreated to prevent scaling of the heating equipment (not shown) and the well 4 as well as the pore space of the formation 1. Further agents maybe added, such as for'preven ting the formation of precipitates when contacting the formation water. Thermal energy of the fluids recovered via the well 5 may be used for heating the aqueous fluid. 1

The method according to the invention is not restricted to the use' in a field in which only two wells penetrate the formation. Any other number of wells, which may be arranged according to any pattern, may be applied in using the present method. Further, it should be understood that various modifications, alterations, and adaptations may be applied to the disclosed approach to meet the requirements of practice without in any manner departing from the spirit of the invention or the scope of the subjoined claims.

We claim as our invention:

1. A method of recovering crude oil having a viscosity above 50 cp. from a subsurface formation having an oilsaturated zone overlying a water-saturated zone, said method including the steps of: I

injecting an aqueous fluid having in its liquid phase a density greater than the density of the crude oilinto the water-saturated zone of the subsurface formation through an injection well; injecting a solvent suitable for dissolving crude oil and having in its liquid phase a density which is smaller than the density of the crude oil into the water-saturated zone at least partially simultaneously withthe step of injecting the aqueous fluid therein; and producing from said subsurface formation fluids includ ing said crude oil through a second well.

2. The method according to claim 1 wherein the aqueous fluid is injected into the injection well at ambient temperature. j

3. The method according to claim 1 wherein at least part of the aqueous fluid is heated before being injected into the injection well.

4. The method according to claim 3 wherein the aqueous fluid at least'partly consists of steam. I

5. The method according to claim 3 wherein the aqueous fluid consists at least partly of hot water, which is at least partly'replaced by steam when at least part of the crude oil;; has been recovered from the formation.

6. The method according to claim 1 wherein the level at which the -wells open into the formation is shifted in an upward direction to a level near the interface between the oil-saturated zone and the water-saturated zone of the formation.

7. The method according to claim 1 wherein the injection of solvent is interrupted after the lower layer of the oil-saturated zone has obtained a reduced viscosity.

8. The method according to claim -1 wherein the injection of solvent is periodically interrupted after the lower layer of the oil-saturated zone has obtained reduced viscosity.

9. The method according to claim 1 wherein the'sol vent and the aqueous fluid are injected into the formation in the form of an unstable emulsion.

10. The method according to claim 1 wherein the solvent and the aqueous fluid are injected into the formation in the form of an unstable emulsion, the instability of the emulsion being decreased in the later stages of the method.

(References on following page) References Cited UNITED STATES PATENTS Ranney 166--269 Dixon 166-272 Lindauer 166--275 X 5 Draper et a1. fin- 166272 Holbert: 166269 X Binder ct a1 166-275 X 8 3,170,513 2/1965 Dew et a1. 166-2 74 3,208,517 9/1965 Binder et a1. 166- 2 4 3,369,601 2/1968 Bond et a1. 166-258 STEPHEN I. NOVOSAD, Primary Examiner U.S. C1. X.R, 166274, 275 

